1. Technical Field
The present invention is a method for a rapid verification of satisfactory clean-up at the end of the completion process when starting production of petroleum fluids while the completion rig is still at the well location. More specifically, the method concerns marking of fluids by one or more distinct tracers, in one or more separate zones along the well, before start up of the production, followed by registration of back produced tracers to determine whether all the completion fluid has been back produced and if possible determine which well zones are participating to the production in this early production stage. If the verification is positive the well is satisfactory cleaned up and the completion rig may be removed. If the verification is negative and the problems are identified one may potentially use the rig to take immediate actions.
2. Description of the Related Art
During completion of oil wells and the following clean-up during start up of oil wells, it is very important in an as early a stage as possible to have the control on effectiveness of the clean-up process. The costs of having a rig or a drilling vessel at location during this operation is very high, and may generally come to 363,720-606,200 USD per 24 hours. The clean-up process is successful if all completion fluids and particles as cuttings are out of the well and that the anticipated producing zones actually are producing, i.e. oil is flowing into the well in all zones or that it is reasonable to expect that all the zones will generate production.
Clean-up is defined as a process where completion fluids and particles are produced out.
The in-flow properties (skin) along the downhole target will change dramatically during this phase. The process may have several purposes, but the most important are:
a) as far as possible to prove that all completion fluid and particles are out
b) as far as possible to verify that the “mudcake” has been broken down, and that the well is flow connected to a desired degree to the reservoir
c) give an indication about the inflow profile
To verify that the clean-up process has given a satisfactory result within a short time, gives the possibility to “release” the rig or the drilling ship at an as early as possible stage and secure that one may not have to carry out clean-up actions that is both time consuming and gives unnecessary high costs as a result, on a later stage.
A verification of the clean-up process may be performed by the use of chemical compounds as for instance substances that may be detected by use of spectroscopic methods such as UV, VIS, Fluorescence, and so on. It is known that produced oil contains different components that may cause interference with the tracers due to its own absorption maxima in the UV and VIS range. The detection methods of today are complicated and time consuming in proportion to the time pressure existing on releasing the rig.
WO2010005319A1 describes a tracer system wherein the tracer components comprise polyether alcohol or derivatives thereof.
These tracers are specially arranged to be detected in very low concentrations. The method for analysis is complex.
US 2006/0144588A1 discloses a portable arrangement for analysis of placed tracers of a certain concentration to an oil well. The analysis is quantitative with regards to concentration.
WO0181914A1 describes an intelligent release of tracers to decide where, i.e. in which zones, water, oil or gas flows into the production well, and for following use of that information to possible closing off zones, mapping of production rates or in other ways controlling hydrocarbons to give a more optimal reservoir exploitation over years.
The publication SPE 135070 “Using Chemical Tracers for Flow Profiling a Subsea Horizontal Well . . . ” by Bakhshi et al. describes a study which includes placing different tracers in specific locations along the length of a lower completion. The tracers may be soluble in oil or water. Samples are taken at the surface during a short period of start up. The samples are analyzed to determine the presence of tracers. Based on the results and the known placed position qualitative information of the fluid flow in the well is generated. All analysis of tracers are carried out on shore.
The publication SPE “Parmanent Tracers Embedded in Downhole Polyners Prove Their Monitoring Capabilities . . . ” by Fridtjof Nyhavn and Anne D Dyrli describes a well completion with placed tracers along the lower part of the completion. The tracers are designed to change behavior as a function of the surrounding environment. Released tracers are analyzed from samples taken topside and the concentration of the tracers gives information of the well flow. The tracers are soluble in oil. Sampling is carried out over an extended period and all the analyses are carried out on shore and are time consuming, up to a week.
The prior art describes methods of collecting well flow information by the use of different types of tracers. The analysis is time consuming and complex and performed in a separate laboratory mostly on shore. Since the day cost of having the completion rig at the well site is very high, reduction of the time of analyzing is essential.
US 2005/0109087 A1 describes methods and compositions for determining the source of fluids or particulates for subterranean formations and the tracer matrix that may be used within such methods. Different tracers are to be placed in different zones and the photoactive material used is detected in the return flow. Detection of tracers may be carried out by detection techniques that can measure the absorption or emission of light. Methods referred are by calorimeters, thin layer chromatography, gas chromatography, HPLC, UV-, VIS-, fluorescence- and mass-spectroscopy. Due to the fact that produced oil contains different components that may cause interference with the tracers due to its own absorption maxima in the UV and VIS range detection in oily samples may be difficult. This patent application describes the same technology and solution as WO0181914 A1 above, but has a more detailed description